Mask ROMs, a kind of nonvolatile memory, is the device which a variation is impossible after the fixation of data at a stage of manufacture, and is utilized in the use of ROM for a character font of CRT display or printer, and in the use of data programming directed by a user.
Mask ROMs has a simple memory cell structure so that it has higher integration density than any other semiconductor memory, and a large amount of ROMs of the same data ordered by a user can be manufactured at the same time so that the cost is reduced, so it is widely used.
FIGS. 1A to 1D are cross-sectional views showing manufacturing process of masks ROMs according to the conventional technique.
Referring to FIG. 1A, a gate insulating layer 3 and a poly-silicon gate 5 are first formed on a P-type silicon semiconductor substrate 1 by the known method, and then N-type impurity ions, such as phosphorous and arsenic, opposed to the substrate 1 are injected into the substrate 1 to form a first ion implantation layer 7.
Referring to FIG. 1B, after the formation of a spacer 11 on the sidewall of the poly-silicon gate 5 by the known method, ions of the first ion implantation layer 7 is activated by means of a thermal treatment so as to form source and drain regions 8 and 9. subsequently, N-type impurities, such as phosphorous and arsenic, are injected through the entire surface of the structure so as to form a second implantation layer for the ohmic contact between the source and drain regions 8 and 9 and metal lines. Ions of the second ion implantation layer is next activated by means of a thermal treatment so as to form source and drain diffusion regions 13 and 15.
At this time, it should be noted that the energy of the activated ions is so suitably controlled as not to pass through the source and drain regions 8 and 9.
The substrate 1 with the abovesaid components is stored in an ambient state of N.sub.2 or a vacuum state container until the following steps being performed according to the desired program.
Referring to FIG. 1C, if desired, programming is performed on the stored substrate 1 according to the desired program. After forming a photoresist layer pattern 16 by the known photo- lithography method to expose a programming region 14, P-type impurity ions are injected in high energy enough to pass through the poly-silicon gate 5 of the programming region 14 and the gate insulating layer 3 so as to form a third ion implantation layer 17.
The impurity ions injected into parts except for the lower part of the gate insulating layer 3 don't affect the junction because passing through the exposed source and the drain regions 8 and 9 of the programming region 14 due to high energy.
Referring to FIG. 1D, after the removal of the photoresist layer pattern 16, a protection layer 19 and metal interconnections are formed by the known method, and then a passivation layer 21 is formed of glass insulating material, such as BPSG(Boro-Phospho Silicate Glass), PSG(Phospho Silicate Glass), USG(Undoped Silicate Glass), on the entire surface of the structure. At this time, the thermal energy occurred during the abovedescribed processes activates the third ion implantation layer 17 so as to form a channel region 18 under the gate insulating layer 3.
Thus, in the manufacturing method of mask ROMs according to the above described conventional technique, the silicon semiconductor substrate on which the gate insulating layer, the source and drain regions, and the source and drain diffusion regions are formed is stored until the step, such as programming, being performed according to the desired program.
Accordingly, there are occured some problems that the poly-silicon gate and the source and drain regions of the structure can be polluted by organisms or natrium, etc. as they are exposed to air during the long time storage, and the manufacturing time is prolonged. Further, the efficiency and reliability of mask ROMs are degraded because the following steps, for example, programming, are performed by using the polluted substrate.
Moreover, the identification of a program takes much time because it is impossible to check the program during programming so that the final mask ROMs is inspected for checking defective chips.